The precursor of Tetrahymena thermophila rRNA can be spliced in the absence of any protein or enzyme in vitro. The splicing reaction is mediated by the IVS (intervening sequence) portion of the RNA. The sequence and structure of the IVS RNAs specify the splicing sites and promote the splicing reaction. Similar self-splicing activity has been identified in two mitochondrial mRNAs. The detailed mechanism of the reactions has not been clarified. Self-splicing RNAs, which have been categorized as Group I, share three sets of conserved sequences that determine the structure of their IVS and three nucleotides that presumably specify their splicing sites. Thus it is plausible that all Group I splicing reactions proceed through a common reaction mechanism. We have proposed a model reaction mechanism based on our secondary structure analysis of the IVS RNA of Tetrahymena rRNA and on the properties of a number of RNA cleavage reactions that are mediated by Tetrahymena IVS RNA. We have identified an intermolecular RNA cleavage reaction that is mediated by the IVS RNA. This enables us to manipulate independently the mediating site in the IVS RNA and the cleavage sites in the substrate RNA. Furthermore, it allows us to investigate new classes of RNA-mediated RNA cleavage reactions. Our research has two objectives. First, we will attempt to identify the minimum sequence and the structure requirements for specific intermolecular cleavage reactions that are relevant to self-splicing. These studies will be a critical test of our model reaction mechanism and may help to establish the mechanism of self-splicing in this class of RNA. Second, we will investigate other specific RNA-mediated cleavage reactions, since we believe that other classes of the specific cleavage reactions may exist. Such reactions would be of great interest to the field of molecular biology, since a variety of RNAs have now been shown to play a significant role in modulating gene expression. We hope that the findings of our research will shed new light on the properties of RNA-RNA interactions that are relevant to gene expression of higher organisms.